Stratigraphy and drilling geomechanics of a thick salt section: the Ariri Formation in the eastern portion of the Santos Basin

Abstract

Borehole instability costs millions of dollars to the oil industry as a consequence of the non-productive time of drilling operations. Stratigraphic and drilling geomechanics studies of salts layers of the Ariri Formation provide offer valuable clues to control the instability referred to above. The study area is located in a layered and thickened zone that was created by a compressive halokinetic deformation. A seismic 2D-section was traced passing through a number of wellbores with the best lateral continuity of the salt layers. Eight wellbores were selected for these studies. Major cycles of salt deposition were identified and named as C1, C2, C3, and C4 from bottom to top of the Ariri Formation. The cycle C1 is the most thickened cycle by the halokinesis and the thickest of them. It is halite rich, seismic transparent to chaotical, and shows low interbedded layers. The C2 and C3 cycles have similar geologic and seismic facies and shows strong signal of interbedded layers of anhydrite, halite, and bittern salts. The C4 cycle has chaotical seismic facies and strong reflections that are related to the greater quantity of bittern-salt and anhydrite layers. Salt creeping in the halite layers of the C1 cycle is the main cause of borehole instability. Salt creeping can lead to Bottom Hole Assembly (BHA) stuck, which requires additional services and inflate both operation time and costs. Borehole pressures and temperatures while drilling under salt creeping conditions that produced the BHA stuck were used to draw a Scatter Plot. Statistical analyses have demonstrated a good correlation between pressure and temperature values representing a salt creeping condition that favors BHA stuck. Borehole instability is also related to bittern-salt drilling. Bittern salts are hydrated K-Mg-chlorides that have similar characteristics: high solubility, low density, and low acoustic velocity. Their occurrences are more restricted in the Santos Basin but even so they can cause serious borehole instability problems. Those salts can be washed-out by drilling fluids that enlarge the borehole diameter uncontrollably. On the other hand, a thick layer of bittern salts can creep fast and cause the borehole closure in a high differential pressure condition.